The present invention relates to an incandescent light bulb (hereinbelow referred to as "lamp") and more particularly, to a light protection arrangement for protecting the light against rush current when the light is first turned on.
When a light is first turned on, a rush of current greater than its rated current flows through the filament of the light additionally, since lights generally exhibit resistance value, (approximately 10 times lower) when they are in use and when they are not in use, the rush of current when the light is first turned on is extraordinarily high with the result that the filament of the light sharply rises in temperature, thereby undesirably shortening the life of the light due to thermal fatigue of the filament.
Accordingly, it has been conventionally proposed that a negative temperature coefficient (NTC) thermistor 3 be connected in series between a power source 1 and a light 2 as shown in FIG. 1 so as to lengthen the life of the light 2. Specifically, since when a switch 4 is initially closed the light 2 has a small resistance value (approximately 10% of the rated value), a large rush of current would flow into the filament of the light 2, if the NTC thermistor 3 is not provided. However, when the NTC thermistor 3 is provided as shown in FIG. 1, the rush of current when the light is first turned on is restricted to a relatively small value, and the light 2 reaches substantially steady brightness after a predetermined time period as the NTC thermistor 3 is lowered in resistance due to its self-heating. When the rush of current is prevented from flowing into the light 2 as described above, the filament does not rise sharply in temperature, so that the filament is less subjected to thermal fatigue, whereby the life of the light 2 can be desirably lengthened.
While the NTC thermistor may be used in a circuit where power is supplied exclusively to the light, it cannot be used in a circuit where other electrical parts are supplied power together with the light because the NTC thermistor may have a derogatory effect on such electrical parts. For this reason the above-described NTC thermistor has not in practice been applied to the light in spite of its known usefulness.
Accordingly, an essential object of the present invention is to provide an improved light protection arrangement which does not require any additional wiring or special works so as to lengthen the life of a light and prevent damage to the light due to abnormal current, with substantial elimination of the disadvantages inherent in conventional light protection arrangements of this kind.
Another important object of the present invention is to provide an improved light protection arrangement of the above-described type which is simple in structure, highly reliable in actual use and suitable for mass production at low cost.